1. Field of the Invention
This invention pertains generally to sputter coating and, more particularly, to an apparatus and a method for depositing a step coating on a workpiece by sputtering. More particularly still, the present invention relates to an apparatus and method for collimating sputtered material in a semiconductor processing chamber.
2. Background of the Art
Sputter coating is commonly employed in the formation of films on substrates in the manufacture of semiconductor devices, and planar magnetrons have long been used as sputtering devices to coat silicon substrates with various materials, such as titanium and aluminum, during the manufacture of integrated circuits.
With sputter coating, it is difficult to form a uniform thin film, or step coating, which conforms to the shape of a workpiece where a step occurs, e.g. at the upper or lower corner of an opening such as a hole or a via in the surface of a workpiece. As the density of devices on semi-conductor substrates increases and etching technology evolves, the aspect ratio of openings on the surface of semi-conductor substrates correspondingly continually changes such that the depth of an opening can exceed the width as shown in FIG. 1. FIG. 1 shows an opening 20 having an aspect ratio wherein the depth of the opening is approximately twice its width. It is difficult to fill small openings (e.g. one micron, or less, in diameter or width for a 1.2 micron depth) and to provide controlled film growth on the side 22 and bottom 24 walls of such openings. These difficulties arise because particles tend to leave the source in all directions, arriving at the workpiece from a variety of angles as shown in FIG. 2. Particles of escaping target material can leave the surface of the target at any angle and from any surface location as shown by the arrows in FIG. 2. When depositing target material onto a substrate having an opening with a high aspect ratio, the deposited material striking the upper portion of the opening at acute angles tends to close the mouth of an opening before the entire opening is completely filled such that a void 26 is formed within the opening as shown in FIG. 2. Such voids can cause long-term reliability problems in semi-conductor devices.
As shown in FIG. 3, a collimator 12 can be used to resolve such problems. A collimator 12 is a filtering plate positioned between a sputtering source 14 and a semi-conductor workpiece 16. A collimator 12 typically has a finite predetermined thickness and includes a number of passages 28 of predetermined dimensions formed through its thickness, through which the deposited material must pass on its path from the sputtering source 14 to the workpiece 16. The collimator 12 filters out target material that would otherwise strike the workpiece at acute angles exceeding a desired angle. The actual amount of filtering depends upon the aspect ratio of the passages 28 through the collimator 12, such that only deposited material within an angle of [arctan (cell width/cell height)] from the perpendicular can pass through the collimator 12 to strike the workpiece 16. Particles traveling on a path approaching normal to the workpiece 16 pass through the collimating plate 12 and deposit on the workpiece. This allows improved semi-conductor device manufacture for substrate devices formed with high aspect ratio holes, trenches, etc. The aspect ratio of the collimator cells is defined as the cell height over the cell width.
There are several disadvantages to the use of typical collimators. First, each particle of target material which strikes the upper surface 30 of the collimator plate 12 is deposited on the collimator. This can significantly reduce the efficiency of depositing material in a processing chamber because a portion of the target material is deposited on the collimator rather than the intended semi-conductor wafer and thus wastes target material.
Second, the deposited material strikes the sidewalls of the collimator cells when it enters the cell at an angle greater than [arctan (cell width/cell height)] from the perpendicular. Such material is thus deposited within and tends to clog the passages of the collimator. Additionally, material deposits on the planar surface of the collimator facing the target and existing between the individual cells. FIG. 4 illustrates how target material 34 deposits on the cell walls 32 and planar surfaces of a collimator. This deposition of material 34 onto the cell walls 32 increases the aspect ratio of the passages so that less material from the sputtering source 14 will pass through the collimator 12 and be deposited on the workpiece. This increases the throughput time of subsequent substrates which are processed at the same plasma energy to achieve the same thickness of deposited material.
Third, over the life of the sputtering source 14, the effective width of the collimator cells changes as target material 34 is deposited onto the collimator. The buildup of target material 34 on the collimator 12 causes poor film layer uniformities at the end of collimator life as represented graphically in FIG. 5 because a greater buildup of sputtered material on the cell walls will occur at some localized portion of the collimator as compared to the buildup on other portions of the collimator. This occurs because of the geometry of sputtering a planar surface to coat a planar surface. If the rate at which the target is sputtered is equal across the entire target surface, more target material will reach the center of the collimator than will reach its edge where the collimator is centered to the target center. This occurs because the material sputtered from any point on the target will leave the target in a fan-shaped cosine distribution. The material sputtered from the center of the target will deposit on the center of the collimator all the way to the collimator edge, with a declining deposition rate as the collimator edge is approached. Those particles sputtered from the edge of the target are as likely to travel inwardly of that location as outwardly of that location, such that a substantial portion of the material sputtered from the target edge will not reach the collimator or substrate. This results in a greater deposition thickness at the center of the collimator because material reaches it from both the center and edge of the target. This sputtering profile can be changed by placing a magnetron apparatus behind the target to create annular regions of high rate sputtering which are designed to create a more uniform deposition thickness on the substrate, i.e., to compensate for the tendency to over deposit on the substrate center by reducing the sputtering rate at the center of the target as compared to the target edge. However, these magnetron systems must be designed with a specific target to substrate spacing to optimize deposition thickness uniformity. The location of the high sputtering rate region of the target must be related to the fan shaped sputtering distribution and the distance between the substrate and the target. Where a collimator is located intermediate the substrate and target and target erosion is optimized to provide uniform thickness film layer on the substrate, the rate of deposition on the collimator surfaces will vary over the diameter of the collimator.
FIG. 6 depicts how cell width decreases over the life of the sputtering source. Over the life of the collimator, cell width decreases more in the center portion of the collimator relative to the outer portion of the collimator where a uniform erosion of the target is present. The use of standard collimators with uniform cell size and aspect ratio as shown in FIG. 3 often leads to problems of poor film uniformities as a result of a differential thickness layer being formed across the surface of the collimator.
There exists a need for a collimator for use in semi-conductor processing which improves film layer thickness uniformity, both on a substrate and from substrate to substrate. There also exists a need for a collimator which provides a uniform deposition despite varying changes in the cell aspect ratio over the diameter of the collimator during its life.